Injection molding apparatus

ABSTRACT

A molding extruder having a pressure-relieving device which will relieve relatively high pressure to a lower holding pressure within the extruder on a delay after fluid molding material has been fully injected into the cavity of a mold and in which the time of completion of injection into the mold cavity is determined by an inherent reduction of feed speed of the extruder plunger.

United States Patent Shigeo Ichikawa;

Keizo Nagawa, both of Aichi-ken, Japan 861,903

Sept. 29, 1969 Jan. 4, 1972 Kahushiki Kaisha Meikiseisakusho Nagoya-shi,Aichi-ken, Japan [32] Priorities Oct. 1, 1968 [3 3] Japan Oct. 1, 1968,Japan, No. 43/7l,265

[72] inventors [2i 1 Appl. No. [22] Filed {45] Patented [73] Assignee[54] INJECTION MOLDING APPARATUS 6 Claims, 5 Drawing Figs.

[52] US. Cl 425/135, 425/149, 425/207 [51] Int. Cl B29f 1/04 [50] Fieldof Search 18/30 CR,

30 CM, 30 CS, 30 CV, 30 SS, 30 SR, 12 SM, 12 A; 264/40, 328, 329

[56] References Cited UNITED STATES PATENTS 2,926,386 3/1960 Hutchinson264/328 X 3,359,601 12/1967 Evans 18/30 SS 3,371,386 3/1968 Ludwig l8/30CR Primary ExaminerJ. Spencer Overholser Assistant ExaminerNorman E.Lehrer Attorney-John B. Armentrout ABSTRACT: A molding extruder having apressure-relieving device which will relieve relatively high pressure toa lower holding pressure within the extruder on a delay after fluidmolding material has been fully injected into the cavity of a mold andin which the time of completion of injection into the mold cavity isdetermined by an inherent reduction of feed speed of the extruderplunger.

VOLTAGE RELAY DELAY RELAY DETECTOR II 2 b :LEL:

I0a P I I0 1 rmmcum m 35321246 snmlbrz F/GI I VOLTAGE' v RELAY g2 iVOLTAGE- RELAY FIG. 2. V we 'M INJECTION MOLDING APPARATUS The presentinvention relates to a molding extruder, more particularly to a moldingextruder having a pressure relieving device for reducing a pressurewithin the extruder when a fluent molding substance has filled a moldcavity by injection.

It is conventional to arrest the occurrence of shrinkage of injectionmolded products by applying pressure to the product in an injection moldfor a period of time such as until solidification of a gate portion ofthe product occurs. However, it is not desirable in terms of quality offinished products nor from the standpoint of economical powerconsumption to maintain pressure for the latter purpose as high withinan extruder cylinder of the injection press or to operate a hydraulicpump during the shrinkage arrestation interval at the relatively highpressures used for injection. Therefore, it is usual practice tomaintain the pressure within the extruder cylinder at a low valuesufficient to keep the plastic resin in a slightly pressurized conditionafter a substantial completion of the injection of plastic resin intothe cavity such as with switching on a pump having small deliverycharacteristics or through use of large delivery pump substantiallyunloaded. In order to effect changeover in pressure, limit switchesheretofore have been used actuated as the injection plunger has beenadvanced to a predetermined position or pressure changeover has beeneffected based upon the elapse of a predetermined period of timeextending from initiation of the injection plunger movement for fillingthe mold.

Since the abovementioned means are not constructed so as to detect thecompletion of a injection of plastic resin into the cavity of a mold,thereby permitting the injection pressure to be relieved, any highinjection pressure preset to a value for being relieved may, forexample, be maintained for an unduly prolonged period of time even afterthe plastic resin has completely filled the cavity of the mold, thus inthe latter instance resulting in a high residual stress within thefinished product. Also, means have been provided within the cavity of amold for detecting a rise in pressure at the time when the mold cavitybecomes full, but placement accordingly mars the mold face and leads toa scar in the surface of the molded product. Also, since the detectingmeans under these conditions has been provided through production of themold, installation has been complicated.

In addition, as a simple means, a pressure switch has been used fordetecting the pressure within the extruder cylinder and therebyrelieving the extruding pressure. However, a pressure switch oftentimeshas proved to be inadequate, for detecting the instant when the moltenplastic resin has filled the mold cavity, for such reasons as beingprone to any one or more such deficiencies as inability to maintain apredetermined pressure response characteristic, frequent introduction ofmalfunctions, and poor followup to a change in pressure.

An object of the present invention is to provide improvements indetecting the completion of injection of a fluent molding material intoa mold cavity and in relieving the injecting pressure.

In certain practices in accordance with the present invention, it isobserved that a resin plasticized, for example by means of a screwplunger within a heating cylinder of a molding extruder, and injectedinto the cavity of a metallic mold as by the advancement of the screwplunger, starts to cool down in the mold and thus encounters an increasein resistance to flow. Moreover, the pressure within the extrudercylinder will start to increase. When the screw plunger is further urgedin the forward direction after the plastic resin has filled the moldcavity, the pressure within the cavity, which was attributable toresistance to flow, will now tend to compress the plastic resin, therebyproducing a sudden rise in pressure. That is, the pressure within thecavity, which was caused on flow until then, will increase to a staticpressure.

The pressure within the cylinder of the extruder will be raised as thatof the plastic resin increases, about the time when the pressure withinthe mold cavity is converted from pressure due to flow to staticpressure. Meanwhile, the delivery rate of oil, from a pump, energizingthe screw plunger rectilinearly, will decrease as speed of advancementof the screw plunger is reduced.

Another object of the present invention is to provide in an injectionmolding apparatus a device for detecting the retardation or reduction inspeed of advancement of a plunger as the plunger injects fluent moldingmaterial into a mold and causes the mold cavity to become full, andincorporating means for transmitting a signal from the detecting devicerepresenting that the mold is full and reducing pressure energizing theplunger to a lower pressure in response to the signal after a delay,thereby maintaining relatively high pressure in the mold to assure thatthe mold is full and thereafter exerting a lower pressure.

In accordance with the present invention, there is provided a moldingextruder comprising, a plunger for feeding molten plastic resin into thecavity of a metallic mold and detector means for detecting a retardationof the feed speed 'of the plunger, the retardation of feed speed of theplunger beingincidental to substantially the completion of injection ofthe plastic resin into the cavity, a change-over valve for relievingpressure in the mold to a smaller value, means for transmitting a signalto actuate the changeover valve when a detected retardation in speed asappraised by the detector means reaches a predetermined value, and relaymeans for delaying the arrival of the signal at the changeover valve.

Preferred embodiments according to the present invention will bedescribed in detail by way of example with reference to the accompanyingdrawings, in which FIG. 1 is a schematic view partly in section showingone embodiment according to the present invention,

FIGS. 2 and 3 are side elevation views in enlarged scale showing twodifferent embodiments of the speed detecting means for the screwplunger,

FIG. 4 is a wiring diagram showing connections of wirings used in FIG.3, and

FIG. 5 is a schematic view partly in section representing a furtherembodiment herein.

Referring now more particularly to FIG. 1, a suitably heated cylinder 6of an extruder is shown having a screw plunger 4 fitted therein andconnected to a metallic mold 8 with a front chamber 7 thereof incommunication with a cavity 9 of the metallic mold 8. The plunger 4includes a piston 3 at one end portion opposite to that having a screwthread. The piston 3 is reciprocatively fitted into a cylinder 5 locatedat one of the heated cylinder 6. An oil pump 1 is connected to cylinder5 through a fourway valve 2 so as to supply oil to either side of thepiston 3 selectively. A relief valve 10 is connected to the deliveryline of the pump 1. A secondary relief valve 12 is connected to thedrain line 10a which is branched off from the delivery line of thepump 1. The secondary relief valve 12 is usually set at a pressure lowerthan the set pressure of the relief valve 10. A speed sensing meansdesignated A is mounted on the cylinder 5 so as to detect a change inspeed of the screw plunger 4 and then transmit a signal to a solenoid11a of twoway valve 11 via a voltage relay 14 and a delay relay 13. Theabovementioned drain line 100 from the delivery of the plump l, as shownin FIG. 1, is led to pass connecting the twoway valve 11 and thesecondary relief valve 12 in series and is tapped into the secondaryrelief valve for the latter to be responsive to the drain line pressureat the aforementioned set pressure.

When the pump as shown in FIG. 1 is actuated, with the fourway valve 2meanwhile being placed in communication, through side designated a, soas to supply working oil from the pump to chamber 5a of the cylinder 5,thereby displacing the piston 3 and hence the screw plunger 4 forwardlyor to the left, the resin which is already plasticized and collected inthe chamber in front of the screw plunger 4 in the heated cylinder 6will start to flow into the cavity 9 of the metallic mold. As

plunger 4 is advanced farther, the plastic resin will eventuallycompletely fill the cavity.

As described hereinabove, the speed of the screw plunger will beconspicuously retarded or reduced about the time when the pressure ofthe plastic resin within the cavity is converted from pressure underflow to static pressure. Accordingly, a determination of the instantwhen the cavity is substantially full of molten plastic resin is bad bydetecting the reduction in feed speed of the screw plunger.

In one embodiment according to the present invention, the speed isdetected by the speedsensing means A which will transform the speed intoa voltage signal. When the signal transmitted from the speed sensingmeans A reaches a value preset at the voltage relay 14, the solenoid 11aof the twoway valve 11 will be actuated through the delay relay 13.Then, the drain line a from the delivery side of the pump 1 will becommunicated to the secondary relief valve 12 through the twoway valve11, thereby relieving the pressure introduced in the cylinder 5 throughvalve 2 in the side a communicating position. The delay relay isdesigned to cause the operation of the twoway valve 11 to be delayedminutely by delaying the signal transmitted from the voltage relay 14.

Toward avoiding undue prolongation of relatively high pressureconditions in the mold before fluid energizing pressure on the screwplunger 4 can be alleviated to a holding pressure through opening of thetwoway 11, a particular retardation in speed of the screw plunger asdetected by the speed sensing means A and transmitted to the presetvoltage relay 14 is determined to be appropriate for such an avoidance.Also, to assure feed of plastic into the mold in quantity adequate forfilling the mold according to the corresponding detected retardation inspeed of the screw plunger, a sufficiently accelerated speed of drivingthe screw plunger is arrived at and reliance for minute additionalsupply assurance is placed upon use of the delay relay 13.

As a result of delay of the time of relieving the extruding or injectingpressure under the dictates of the delay relay 13, the pressure withinthe extruder cylinder on operation of the pump 1 will be raiseddependently upon the period of time during which relief is delayed. Ifneed be, feed of a small amount of the plastic resin under this pressurerise of short duration will occur into the mold cavity to compensate forinsufficient feed. If a delay relay which is adjustable in terms of apreset time is used, it will be possible to assure minute adjustment.

Now, with reference to FIG. 2, a stationary rack 103 is fixedly mountedon a frame of the molding extruder. A pinion 102, which is meshed withthe rack 103, is provided connected so as to move in unison with thescrew plunger 4 in a rectilinear direction without rotating with thescrew plunger. A tachometer also is provided so as to reciprocatetogether with the screw plunger. The rack 103 and pinion 102 willindicate the linear motion of the screw plunger 4 in number ofrevolutions. The revolution of the pinion 102 is transmitted to anamplifying gear 104, which in turn is connected with the tachometer 101,by means of a belt 105. A timer 106 is connected so as to inhibitoperation of the voltage relay 14 on control of the tachometer outputuntil after the speed of the screw plunger 4 attains a predeterminedvalue during initial rectilinear in stroke advance of the screw plunger.When the output of the tachometer 101 later in stroke of the screwplunger 4 becomes smaller than the value presetat the voltage relay 14,the latter will work to relieve the injecting pressure by energizing thesolenoid 11a of the twoway valve 11.

With reference to FIGS. 3 and 4, a photoelectric switch PH is providedso as to move in unison with the screw plunger 4. A slit plate 201,which is fixedly mounted on the frame of the molding extruder, islocated between a light source L and a receiver element J of thephotoelectric switch PH, see FIG. 4. As shown in FIG. 3 slits 201a areprovided at uniform spacing on the slit plate 201 only in the leftsidehalf thereof.

Now, the screw plunger 4 will be accelerated when it is advanced tostart extrusion. However, in the interests of inhibiting both operationof the photoelectric switch PH and operation of the voltage relay 14early in stroke of the screw plunger 4, no slits are provided on theslit plate in the rightside half thereof. As the photoelectric switch PHadvances from each slit portion 201a of the slit plate 201 encounteredto the next, with advance of the screw plunger 4, the light beamemanated from the light source L of the photoelectric switch PH isinterrupted by an intermediate portion of the plate 201. The receiverelement J, as controlled by the light beam, has a predetermined speedresponse making it possible to have the photoelectric switch not operatewhen the speed of the screw plunger is greater than the response speedof the receiver element J. When the pressure of the plastic resin in thecavity is raised and the speed of advance of the screw plunger isretarded to the response speed of the receiver element J of thephotoelectric switch PH, contacts RA of the voltage relay 14 will beselfretained and the voltage relay will operate, after an elapsed periodof time preset by the delay relay 13, to energize the solenoid 11a ofthe twoway valve 11 to relieve the extruding pressure.

FIG. 5 shows another embodiment according to the present invention. Arack 103 fixedly mounted on the frame of the molding extruder and apinion 102 moving in unison with the screw plunger 4 are used to convertthe linear movement of the plunger into rotation of the pinion. A beltis entrained over the pinion 102 and imparts an accelerated rotation toa tachometer 101 which will transmit a signal dependent upon revolutionsreceived. Output of the tachometer 101 is introduced into a differentialcircuit 15 which will transmit a signal representing the speed of thescrew plunger 4. When the signal transmitted from the differentialcircuit 15 reaches a value preset by the voltage relay 14, electriccurrent will flow through the delay relay 13 to energize the solenoid11a of the twoway valve 11, thereby connecting the drain line 10a fromthe delivery side of pump 1 with the secondary relief valve 12 torelieve the extruding pressure in the cylinder 5.

A few embodiments according to the present invention have been describedhereinabove. It is evident that the extruder means need not berestricted to the screw plunger type and that the present invention mayreadily be practiced through use of an extruder of socalled plungertype.

It further will be seen that in accordance with the practice of thepresent invention, the time when plastic resin has filled the cavity ofthe mold is determined accurately by means as described in detailhereinabove, and that relatively high pressure on the plastic resin inthe mold cavity is not unduly prolonged, thereby giving a molded productof good quality having a minimum amount of residual stress.

Since in essence the pressure of plastic referred to herein is detectedin the mold cavity, products comprising the molded plastic resin fromthe mold tend toward being of constant density and hence constantweight. Also, since the advancement of the screw or plunger is stoppedwhenever the mold cavity is full, the stroke of the screw or plunger isautomatically prescribed. Again, since the advance of the screw orplunger will be stopped whenever the mold cavity is full, it becomespossible to carry out injection regardless of tolerable variations inthe amount of resin injected.

As many possible embodiments of the invention may be made and as manypossible changes may be made in the embodiments hereinbefore set forth,it will be understood that all matter described herein is to beinterpreter as illustrative and not as a limitation.

We claim:

1. In injection molding apparatus, the combination which includes, aplunger, speed sensing means for producing a signal representing apredetermined rectilinear feed speed retardation of said plungercoincident with the substantial completion of filling a mold cavity withfluent molding material on operation of said plunger, pressure reliefvalve means including a valve responsive to said signal for relievingpressure of fluid urging said plunger rectilinearly toward feedingfluent molding material into a mold cavity after responding to saidsignal, and signal transmitting means communicating with said speedsensing means and with said signalresponsive valve and including delaymeans for delaying response of said signalresponsive valve to saidsignal for a brief interval of time following the initiation of saidsignal.

2. In injection molding apparatus, the combination as set forth in claim1 characterized by said plunger carrying a first portion of said speedsensing means for moving said first portion rectilinearly in unisontherewith, and a second portion of said speed sensing means beingdisposed in a related fixed position so that said first portion and saidplunger move in unison rectilinearly past said second portion, one ofsaid portions including a photoelectric switch and the other of saidportions including a slit plate.

3. In injection molding apparatus, the combination as set forth in claim1 wherein signalinhibiting means in the apparatus inhibits signal outputby said speed sensing means during initial rectilinear in stroke advanceof said plunger toward feeding fluent molding material into a moldcavity.

4. In injection molding apparatus, the combination as set forth in claim17 wherein said speed sensing means includes tachometer means fortransforming said rectilinear movement of said plunger into revolutionsrepresenting rectilinear speed.

5. In injection molding apparatus, the combination as set forth in claim17 wherein said delay means for delaying response of saidsignalresponsive valve to said signal includes delay relay means, and avoltage relay is interconnected with said speed sensing means and saiddelay relay means for transmitting a signal energizing said delay relaymeans when output of said speedsensing means reaches a predeterminedmagnitude.

6. In injection molding apparatus, the combination which includes, aplunger, speed sensing means for producing a signal representing apredetermined rectilinear feed speed retardation of said plungercoincident with the substantial completion of filling a mold cavity withfluent molding material on operation of said plunger, pressure reliefvalve means including a signalresponsive valve and pressureresponsivevalve for relieving pressure of fluid urging said plunger rectilinearlytoward feeding fluent molding material into a mold cavity down to alower pressure level in response to pressure of said fluid after saidsignalresponsive valve responds to said signal, and signal transmittingmeans communicating with said speed sensing means and with saidsignalresponsive valve and including delay means for delaying responseof said signalresponsive valve to said signal for a brief interval oftime following the initiation of said signal.

1. In injection molding apparatus, the combination which includes, aplunger, speed sensing means for producing a signal representing apredetermined rectilinear feed speed retardation of said plungercoincident with the substantial completion of filling a mold cavity withfluent molding material on operation of said plunger, pressure reliefvalve means including a valve responsive to said signal for relievingpressure of fluid urging said plunger rectilinearly toward feedingfluent molding material into a mold cavity after responding to saidsignal, and signal transmitting means communicating with said speedsensing means and with said signal-responsive valve and including delaymeans for delaying response of said signal-responsive valve to saidsignal for a brief interval of time following the initiation of saidsignal.
 2. In injection molding apparatus, the combination as set forthin claim 1 characterized by said plunger carrying a first portion ofsaid speed sensing means for moving said first portion rectilinearly inunison therewith, and a second portion of said speed sensing means beingdisposed in a related fixed position so that said first portion and saidplunger move in unison rectilinearly past said second portion, one ofsaid portions including a photoelectric switch and the other of saidportions including a slit plate.
 3. In injection molding apparatus, thecombination as set forth in claim 1 wherein signal-inhibiting means inthe apparatus inhibits signal output by said speed sensing means duringinitial rectilinear in stroke advance of said plunger toward feedingfluent molding material into a mold cavity.
 4. In injection moldingapparatus, the combination as set forth in claim 17 wherein said speedsensing means includes tachometer means for transforming saidrectilinear movement of said plunger into revolutions representingrectilinear speed.
 5. In injection molding apparatus, the combination asset forth in claim 17 wherein said delay means for delaying response ofsaid signal-responsive valve to said signal includes delay relay means,and a voltage relay is interconnected with said speed sensing means andsaid delay relay means for transmitting a signal energizing said delayrelay means when output of said speed-sensing means reaches apredetermined magnitude.
 6. In injection molding apparatus, thecombination which includes, a plunger, speed sensing means for producinga signal representing a predetermined rectilinear feed speed retardationof said plunger coincident with the substantial completiOn of filling amold cavity with fluent molding material on operation of said plunger,pressure relief valve means including a signal-responsive valve andpressure-responsive valve for relieving pressure of fluid urging saidplunger rectilinearly toward feeding fluent molding material into a moldcavity down to a lower pressure level in response to pressure of saidfluid after said signal-responsive valve responds to said signal, andsignal transmitting means communicating with said speed sensing meansand with said signal-responsive valve and including delay means fordelaying response of said signal-responsive valve to said signal for abrief interval of time following the initiation of said signal.